Measured variation of thermal boundary-layer thickness with prandtl number for laminar natural convection from a vertical uniform-heat-flux surface

This investigation presents systematic measurements of the variation of the thermal boundary-layer thickness with Prandtl number for a Prandtl number range of from 0.703 to 8940 for laminar natural convection from a vertical, uniform-heat-flux surface. Experimental measurements of the thermal boundary-layer thickness, using a differential thermocouple probe, were made in five fluids: air, water, ethylene glycol, tri-ethylene glycol, and SF-97-1000 silicone fluid. In addition, a Schlieren system was used to visualize the thermal boundary-layer region in each fluid. Very good agreement was found between the experimental results and the thermal boundary-layer thickness predicted by both the well-known similarity solution and the recent correlation of Churchill and Ozoe. The largest disagreement was found for the highest Prandtl number fluid. For the high Prandtl number silicone fluid the temperature field was also determined for several values of heat flux. While good agreement with theoretical results was found near the surface, somewhat higher temperatures were measured near the edge of the thermal boundary layer. The larger difference at high Prandtl number is attributed to the non-boundary-layer nature of the velocity field and the associated disturbance of the flow by the presence of the test section boundaries.